import multiprocessing
import time, sys


def _target(queue, function, *args, **kwargs):
    """Run a function with arguments and return output via a queue.

    This is a helper function for the Process created in _Timeout. It runs
    the function with positional arguments and keyword arguments and then
    returns the function's output by way of a queue. If an exception gets
    raised, it is returned to _Timeout to be raised by the value property.
    """
    try:
        queue.put((True, function(*args, **kwargs)))
    except:
        queue.put((False, sys.exc_info()[1]))


class Timeout:

    """Wrap a function and add a timeout (limit) attribute to it.

    Instances of this class are automatically generated by the add_timeout
    function defined above. Wrapping a function allows asynchronous calls
    to be made and termination of execution after a timeout has passed.
    """

    def __init__(self, function, limit):
        """Initialize instance in preparation for being called."""
        self.__limit = limit
        self.__function = function
        self.__name__ = function.__name__
        self.__doc__ = function.__doc__
        self.__timeout = time.time()
        self.__process = multiprocessing.Process()
        self.__queue = multiprocessing.Queue()

    def __call__(self, *args, **kwargs):
        """Execute the embedded function object asynchronously.

        The function given to the constructor is transparently called and
        requires that "ready" be intermittently polled. If and when it is
        True, the "value" property may then be checked for returned data.
        """
        self.__limit = kwargs.pop('timeout', self.__limit)
        self.__queue = multiprocessing.Queue(1)
        args = (self.__queue, self.__function) + args
        self.__process = multiprocessing.Process(target=_target,
                                                 args=args,
                                                 kwargs=kwargs)
        self.__process.daemon = True
        self.__process.start()
        self.__timeout = self.__limit + time.time()
        while not self.ready:
            time.sleep(0.01)
        return self.value

    def cancel(self):
        """Terminate any possible execution of the embedded function."""
        if self.__process.is_alive():
            self.__process.terminate()
        raise Exception("Timeout")

    @property
    def ready(self):
        """Read-only property indicating status of "value" property."""
        if self.__timeout < time.time():
            self.cancel()
        return self.__queue.full() and not self.__queue.empty()

    @property
    def value(self):
        """Read-only property containing data returned from function."""
        if self.ready is True:
            flag, load = self.__queue.get()
            if flag:
                return load
            raise load
